In the past, ceramic-made honeycomb structures have been used as a diesel particulate filter (DPF) for capturing fine particles present in automobile exhaust gases, particularly diesel fine particles.
Generally, these honeycomb structures comprise porous partition walls which form, by surrounding, a plurality of cells extending between the two end faces of the honeycomb structure, and an outer wall formed integrally with the partition walls; and, in the honeycomb structures, a plugged part is formed at one end of each cell so that each end face of the honeycomb structure shows a checkerwise plugging pattern with the two patterns being complementary to each other.
FIG. 5 is a plan view which partially shows the end face of such a honeycomb structure having plugged parts are formed (a plugged honeycomb structure); and FIG. 6 is a partial sectional view which shows an example of the c-c section structure of FIG. 5. As shown in these figures, it is ordinary in a conventional, plugged honeycomb structure 21 that, in the partial cells 5 of outermost peripheral portion, having an incomplete cell section, which are located at the outermost peripheral portion of the honeycomb structure and are in partial contact with the outer wall 4, as well as in the complete cells 6 having a complete cell section, other than the partial cells 5 of outermost peripheral portion, a plugged part 8 is formed at one end of each cell and each end face of the honeycomb structure shows a complete checkerwise pattern (a houndstooth check).
When, as above, a plugged part is formed in plugging-necessary cells and no plugged part is formed in plugging-unnecessary cells, in all the cells including the partial cells 5 of outermost peripheral portion, the resulting honeycomb structure, when used in a DPF, etc., can provide a maximum effective filtration area when used as a filter. It is deemed to be ideal. However, the partial cells 5 of outermost peripheral portion are in contact with the outer wall 4 of the honeycomb structure and, as compared with the complete cells 6, have an irregular shape (a partially-chipped shape) and are small in area; therefore, formation of plugged part 8 is difficult in the partial walls 5. Particularly in those partial cells 5 of outermost peripheral portion having a very small area of less than 5% of that of each complete cell 6, formation of plugged part 8 needs much trouble and labor.
In order to reduce such labor and increase the mechanical strength of honeycomb structure, there is known a honeycomb structure in which the partial cells of outermost peripheral portion are plugged with a heat-resistant material or with the same material as for the honeycomb structure, or the partial cells are plugged in the entire length and formed integrally with the outer wall (see, for example, Patent Documents 1 and 2). There is also known a honeycomb structure in which not only the partial cells of outermost peripheral portion but also part of the complete cells present inside to the partial cells are plugged (see, for example, Patent Document 3). However, when not only the partial cells of outermost peripheral portion but also part of the complete cells, inside to the partial cells are plugged and formed integrally with the outer wall, there have been problems of large reduction in effective filtration area and increase in pressure loss, when used in a DPF, etc.
Patent Document 1: JP-B-1988-12658
Patent Document 2: JP-A-1995-246341
Patent Document 3: JP-A-2004-154768